Method and composition for coating molds



States finite The present invention is directed to a method of and compositions for coating the inner surfaces of molds which are to receive molten metals at a high temperature. The invention is particularly applicable to the coating of metal molds employed for casting steel and other ferrous alloys.

When steel ingots are cast within a metal mold, molten steel is frequently splashed onto the sides of the mold during the pouring of the metal, and the splashed metal solidifies before the remainder of the ingot solidifies. This solidified metal on the sides of the mold may produce surface defects on the surface of the ingot which may manifest themselves as marks, slivers, or seams. The presence of these solidified deposits also gives rise to strains which tear the outer surface of the ingot during its solidification. In addition, the solidified splashed metal remains on the interior surfaces of the mold after the ingot has been stripped from the mold, thereby necessitating careful and time consuming cleaning and reconditioning of the mold prior to its reuse in another casting.

In the past, various means have been employed to provide an interface between the molten metal and the mold Walls in order to minimize the adherence of molten metal splashes to the interior mold surfaces. Frequently, these mold coating compositions had taken the form of liquid coatings such as pitch or tar. However, this type of mold coating composition in necessarily limited in its applicability because at even relatively modest temperatures, the carbonaceous materials will tend to burn or become charred, resulting in the evolution of smoke and odors which are very objectionable. Furthermore, with the use of such materials, it was necessary to cool the mold substantially prior to pouring so that the coating would adhere to the inner surfaces of the mold.

Very substantial improvements in mold coating are achieved through the use of a dry process in which the constituents, in finely divided powder form are applied by means of a spray gun in a stream of air. The coating composition may be applied while the mold is at an elevated temperature so that it is not necessary to cool the mold further after stripping and prior to a second pouring.

' In my previous Patent No. 2,578,976, issued December 18, 1951, I have described compositions which are particularly suitable for thi use. Basically, the compositions there described consisted of a substantial proportion of a starchy material, a resin, and a drying agent of a calcareous nature. While these compositions were very effective under ordinary conditions of use, they some times exhibited a tendency to create significant amounts of slag in the mold. It appears that this slag formation may have been due to the presence of the calcium in the compositions.

The present invention represents the distinct improvement upon the compositions and methods described in my aforementioned previous patent. With the compositions of the present invention, I can coat molds over a wider range of temperatures both higher and lower than previously. The principal reason for this is the fact that I include an improved low temperature binder in the new composition. More important, however, is the fact that the compositions of the present invention are stable at temperatures even beyond those which could be ematent Q ployed with the compositions of the previous invention, without the attendant difliculties of significant slag formation because the newly developed compositions are calcium free.

An object of the present invention is to provide an improvide composition for coating the inner surfaces of molds to prevent adherence of splashed molten metal.

Another object of the present invention is to provide an improved method for coating the interior surfaces of a mold over a wide temperature range.

Still another object of the invention is to provide an improved composition for coating metal molds which does not contribute slag forming ingredients to the molten metal contained in the mold.

A further object of the invention is to provide an improved method for application of mold coating compositions to hot molds.

In accordance with the present invention, I employ the combination of a starchy material, a resinous bonding agent, a bituminous composition, and talc in the form of a finely divided mixture.

The preferred compositions of the invention include starchy or farinaceous material containing less than about 15% moisture. Various starches can be employed, but I prefer to employ potato starch, either alone, or in combination with other vegetable starches such as soy bean starch, tapioca starch, or corn starch. The principal function of the starch is to provide a film forming agent for the composition.

In order to achieve the maximum amount of adherence at relatively low temperatures, I prefer to include soy flour in the composition. As in the case of the starchy materials, the soy flour should contain no more than 15% moisture to function properly.

Additional low temperature binding effects are achieved through the inclusion of a resin or gum, particularly a light colored wood or gum rosin having a melting point of more than about 200 F.

Improved bonding at moderately elevated temperatures is provided by adding the bituminous material, of which powdered pitch is the best example. A commercially available powdered pitch material having a melting point of about 250 C. (482 F.) is particularly preferred.

The talc is employed both as a refractory additive to improve the high temperature behavior of the coating, and also as -a drying agent to permit the storage of the coating composition without caking or lumping.

The relative percentages of the various degrees of the composition, together with the range of particle sizes employed for each, is given in the following table:

Ingredient Particle size Percent by weight Starch 200 to 400 mesh 30-60 Soy flour 200 to 250 mesh 1030 esin to 400 mesh 15-35 Bituminous 150 to 400 mesh 10-20 Talc l 200 to 400 mesh 5-10 For best results, it is important that the total moisture content of the composition not exceed 8% by weight.

The ingredients are blended into the sprayable composition by thoroughly blending the dry ingredients, and then passing the mixture through a 150 mesh screen. The material may then be applied to the interior surfaces of an ingot mold or the like by blowing the composition in powdered form under a positive air pressure into the mold. When the composition contacts the heated mold wall, the binder ingredients melt or soften to adhere to the wall and bind the remaining of the solids to the surface. With the types of compositions indicated above, I have successfully supplied coatings on mold surfaces 3 having temperatures as high as 950 F. although it is preferable, if possible, to work on a mold which has a temperature of about 700 to 750 F. because adherence is at its best in this temperature range.

The following specific examples illustrate the types of formulae coming. within the scope of the invention and the results achieved:

Example I -A coating composition containing the following ingredients was prepared:

' Percent Potato starch Il /2% moisture), 200 to 400 mesh--- 20 Corn starch (7 4% moisture), 200 to 400 mesh 20 Wood rosin, 150 to 300 mesh 30 Soy flour (12% moisture), 200 to 250 mesh 15 Powdered pitch, 150 to 400 mesh 10 Talc, 400 mesh 5 Example I! A slightly modified composition was prepared from the following ingredients:

7 Percent Potato starch (11% moisture), 200 to 400 mesh 35 Soy flour (12% moisture), 200 to 250 mesh 25 Wood rosin, 150 to 300 mesh 20 Powdered pitch, 150 to 400 mesh 15 Talc, 4G0 mesh- 5 The aforegoing composition was particularly effective in coating molds which had a lower surface temperature. As in the case of the material of Example I, the ingot was stripped from the mold cleanly, and the mold itself remained clean.

Example 111 A metal mold was coated with the following composition:

Percent Potato starch (11% moisture), 200 to 400 mesh 50 Soy flour (12% moisture), 200 to 250 mesh 20 Wood rosin, 150 to 400 mesh Powdered pitch, 150 to 400 mesh Talc 5 A steel ingot solidified in the mold coated with the foregoing composition was easily stripped from the mold without significant surface defects, and without impairing the surface of the mold. 7

Example IV Another mold for casting steel ingots was coated with the following composition:

Percent Potato starch (11% moisture), 200 to 400 mesh 30 Soy flour (m /2% moisture), 200 to 350 mesh 30 Wood rosin, 150 to 350 mesh 20 Powdered pitch, 150 to 250 mesh 15 Talc 5 The finely divided mixture was sprayed by means of a conventional spray gun in a current of air onto the inner surfaces of a metal mold used for casting steel. The ingot which resulted was stripped from the mold Without difliculty and the mold surfaces were clean after the stripping. No evidence of additional slag formation was 1. determined due to the presence of the coating composition.

It will be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.

I claim as my invention:

1. A dry, finely divided, air transportable composition for coating the inner surfaces of a mold comprising a finely divided mixture containing the following ingredients:

Percent by weight Starch 30 to 60 Soy flour 10 to 30 Rosin 15 to 35 Pitch 10 to 20 Talc 5 to 10 2. A dry, finely divided, air transportable composition for coating the inner surfaces of a mold comprising a finely divided mixture containing the following ingredients:

Percent by weight Starch 30 to 60 Soy flour a- 10 to 30 Rosin 15 to 35 Pitch 10 to 20 Talc 5 to 10 said composition containing no more than 8% by weight of moisture.

3. A dry, finely divided, air transportable composition for coating the inner surfaces of a mold comprising a finely divided mixture containing the following ingredients:

Percent by weight Starch (200 to 400 mesh) 30 to 60 Soy flour (200 to 250 mesh) 10 to 30 Rosin to 400 mesh) 15 to 35 Pitch (150 to 400 mesh) 10 to 20 Talc (200 to 400 mesh) 5 to 10 4. The method of coating the inner surfaces of a mold having a surface temperature of up to 950 F. which comprises blowing against said surface an air stream carrying a finely divided mixture having the following composition:

Percent by weight Starch 30 to 60 Soy flour 10 to 30 Rosin 15 to 35 Pitch 10 to 20 Talc Sto 10 until an appreciable coating of said mixture appeared on said surface.

5. The method of coating the inner surfaces of a mold having a surface temperature of up to 950 P. which comprises blowing against said surface an air stream carrying a finely divided mixture having the following composition:

, Percent by weight Starch 30 to 60 Soy flour 10 to 30 Rosin 15 to 35 Pitch 10 to 20 Talc 5 to 10 moisture not more than 8%, until an appreciable coating of said mixture appears on said surface.

References Cited in the file of this patent UNITED STATES PATENTS 2,476,933 Wallace July 19, 1949 2,578,976 Ingala Dec. 18, 1951 FOREIGN PATENTS 14,571 Great Britain of 1893 

1. A DRY, FINELY DIVIDED, AIR TRANSPORTABLE COMPOSITION FOR COATING THE INNER SURFACES OF A MOLD COMPRISES A FINELY DIVIDED MIXTURE CONTAINING THE FOLLOWING INGREDIENTS: 